The present invention relates generally to the field of strain measurement devices for fasteners, such as rivets, bolts, and screws, and, more particularly, to a fastener equipped with an untethered fiber-optic strain gauge which is adapted to be coupled to a fiber-optic cable, and a related method of using the same.
There are presently available many different types of sensors and gauges for measuring the strain on fasteners used in mechanical structures in order to monitor the condition thereof. Electrical resistance (or mechanical) strain gauges can be utilized by field personnel to periodically measure the strain on fasteners used in structures subject to deterioration due to corrosion, fatigue, and other sources of mechanical stresses which induce strain. Although such measurements may be scheduled to be taken at regular service intervals, problems may develop between service intervals which go undetected. Further, such measurements are prone to human error, thereby resulting in undetected problems. Moreover, the cost of such servicing over the life of the structure can be very high.
In the case of certain structures, such as military or commercial aircraft, the failure of critical fasteners, e.g., rivets which are used to attach the "skin" to the airframe and to hold the airframe together, can have catastrophic consequences, including the loss of life and of the aircraft. It is therefore imperative that the strain on the rivets be regularly and accurately measured in order to detect failures thereof before they occur, so that the aircraft can be taken out-of-service for the necessary repairs. Obviously, the older the aircraft, the more frequently such strain measurements need to be made, as the likelihood and incidence of fastener failure due to corrosion and metal fatigue increases as a function of time.
A fiber-optic strain sensor has been recently developed by Fiso Technologies, Inc. (Quebec, Canada) which is perfectly linear, can be thermally self-compensated, is not sensitive to transverse strain, and which provides precise, absolute, and stable measurements over long periods of time. This fiber-optic strain sensor is disclosed in U.S. Pat. No. 5,202,939, the disclosure of which is incorporated herein by reference, and in an article entitled "White-Light Interferometric Multimode Fiber-Optic Sensor", Optics Letters, pp,. 78-80 (1993), the disclosure of which is also incorporated herein by reference. This fiber-optic sensor includes a fiber-optic strain gauge which can be embedded into the structure being monitored, e.g., in a bore drilled into a fastener. Such a fiber-optic strain gauge 20 is depicted in FIG. 1.
With reference to FIG. 1, the fiber-optic strain gauge 20 is constructed by inserting a pair of multimode optical fibers 22 into opposite ends of a quartz microcapillary 24. The tips of the multimode optical fibers 22, which form a Fabry-Perot cavity 23, are adjusted to a given cavity length. The optical fibers 22 are then fused directly to the quartz microcapillary 24, the sensitivity being determined by the distance between the welding spots 26, 28, which distance will hereinafter be referred to as the "gauge length" and the region between the welding spots 26, 28 will hereinafter be referred to as the "gauge length region".
When embedded in the structure being monitored, the elongation of the gauge 20 within the gauge length region is completely converted into cavity-length elongation. Thus, the cavity length (i.e., the length of the Fabry-Perot cavity 23) is directly proportional to the strain on the structure being monitored. In the known fiber-optic strain sensor, the fiber-optic strain gauge 20 is coupled to a white light interferometric cavity length measurement system by means of fiber-optic cabling. With this arrangement, the strain on the structure being monitored can be continuously detected by means of the white light interferometric cavity length measurement system. An exemplary white light interferometric cavity length measurement system (referred to as a "white-light cross-correlator") is disclosed in U.S. Pat. No. 5,392,117, the disclosure of which is incorporated herein by reference, and is commercially available as the Fiso Technologies" FTI-100i Series of fiber-optic sensors. The FTI-100i instrument is capable of measuring the absolute Fabry-Perot cavity length of fiber-optic Fabry-Perot strain gauges with a high degree of accuracy, providing highly accurate and reliable measurements.
A large number of such fiber-optic strain sensors can be integrated into a large structure in order to continuously provide information on the state of the structure. A structure equipped with such an integrated network of fiber-optic strain sensors can be thought of as an "intelligent (or smart) structure". Potentially, this technology can be utilized in an aircraft to very accurately and reliably measure the strain on thousands of rivets, screws, bolts, and other fasteners over the lifetime of the aircraft. However, the known fiber-optic strain sensors require that the fiber-optic cabling be routed throughout the entire aircraft to couple each of the thousands of fiber-optic strain sensors to the white light interferometric cavity length measurement system ("reading head"). Consequently, the cost of installation and maintenance of such a fiber-optic strain measurement system would be prohibitively high for this application. Thus, there presently exists a need in the art for a fiber-optic strain measurement technique which overcomes this fundamental shortcoming of the presently available technology. The present invention fulfills this need in the art.